Etching method and etching apparatus

ABSTRACT

An etching method is provided that includes the steps of supplying an etching gas containing a fluorocarbon (CF) based gas into a processing chamber, generating a plasma from the etching gas, and etching a silicon oxide film through a polysilicon mask using the plasma. The polysilicon film has a predetermined pattern and is arranged on the silicon oxide film. The silicon oxide film has at least one of a silicon content per unit volume, a fluorine content per unit volume, and a volume density that varies in a depth direction.

TECHNICAL FIELD

The present invention relates to an etching method and an etchingapparatus.

BACKGROUND ART

In etching contact holes having a high aspect ratio, an etch stop mayoccur at a higher probability as the depth of the hole bottom increases.As a method of preventing an etch stop, for example, Patent Document 1discloses a method that involves increasing the pressure within aprocessing chamber according to the etching progress. According to thismethod, the difference between the pressure within the chamber and thepressure within the holes being formed is reduced as much as possible ateach depth.

PRIOR ART DOCUMENTS Patent Documents Patent Document 1: JapaneseLaid-Open Patent Publication No. 2004-063512 SUMMARY OF THE INVENTIONProblem to be Solved by the Invention

However, in Patent Document 1, the insulating film of a workpiece inwhich the contact holes are formed is made of the same materialthroughout the depth of the holes. Thus, even if the pressure within thechamber is increased in accordance with the etching progress to reducethe pressure differences, it becomes increasingly difficult to reducethe pressure differences as the depth of the contact holes is increased.As a result, an etch stop may inevitably occur. Also, when processconditions such as the pressure are changed, an etching state of a maskmay also be changed. For example, the mask may be deformed or bowing mayoccur at the contact holes, and as a result, the shape of the contactholes may be degraded.

In light of the above, an aspect of the present invention relates toproviding an etching method and an etching apparatus that are capable ofpreventing the occurrence of an etch stop and etching a workpiece into adesirable shape.

Means for Solving the Problem

According to one embodiment of the present invention, an etching methodis provided that includes the steps of supplying an etching gascontaining a fluorocarbon (CF) based gas into a processing chamber andgenerating a plasma from the etching gas, and etching a silicon oxidefilm through a polysilicon mask using the plasma. The polysilicon filmhas a predetermined pattern and is arranged on the silicon oxide film.The silicon oxide film has at least one of a silicon content per unitvolume, a fluorine content per unit volume, and a volume density thatvaries in a depth direction.

According to another embodiment of the present invention, an etchingapparatus is provided that includes a processing chamber and a gassupply source. The processing chamber accommodates a workpiece includinga silicon oxide film and a polysilicon mask, which has a predeterminedpattern and is arranged on the silicon oxide film. The silicon oxidefilm has at least one of a silicon content per unit volume, a fluorinecontent per unit volume, and a volume density that varies in a depthdirection. The gas supply source supplies an etching gas containing afluorocarbon (CF) based gas into the processing chamber. The siliconoxide film is etched by a plasma generated from the etching gas throughthe polysilicon mask having the predetermined pattern.

Advantageous Effect of the Invention

According to an aspect of the present invention, etch stop may beprevented and a workpiece may be etched into a desirable shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of hole shapes according to the etchtime;

FIG. 1B illustrates the etch depth and the etch rate in relation to theetch time;

FIG. 2A is a longitudinal cross-sectional view of a film arranged on asilicon wafer;

FIG. 2B illustrates a relationship between the etch rate and the aspectratio;

FIG. 3 illustrates physical phenomena that occur upon etching a hole;

FIG. 4 illustrates a structure of a film according to an embodiment ofthe present invention;

FIG. 5 illustrates a relationship between the etch rate and the C/Fratio upon etching a silicon oxide film with a fluorocarbon based gasaccording to an embodiment of the present invention;

FIG. 6 illustrates a state of the silicon oxide film upon being etchedusing a fluorocarbon based gas according to an embodiment of the presentinvention;

FIG. 7 illustrates a switching section where layers of the silicon oxidefilm are switched from an upper layer to a lower layer according to anembodiment of the present invention;

FIG. 8 is a longitudinal cross-sectional view of an etching apparatusaccording to an embodiment of the present invention;

FIG. 9 is a lateral cross-sectional view of a film forming apparatusaccording to an embodiment of the present invention;

FIG. 10 is a plan view of the film forming apparatus of FIG. 9; and

FIG. 11 is a lateral cross-sectional view of a film deposition apparatusaccording to an embodiment of the present invention.

EMBODIMENTS FOR IMPLEMENTING THE INVENTION

In the following, embodiments of the present invention are describedwith reference to the accompanying drawings. Note that elements havingsubstantially the same functions or features may be given the samereference numerals and overlapping descriptions thereof may be omitted.

INTRODUCTION

In etching a contact hole (hole) having a high aspect ratio, the etchrate tends to decrease as the depth of the hole bottom increases. Such aphenomenon is described below with reference to FIGS. 1A, 1B, 2A, 2B,and 3. FIG. 1A is a cross-sectional view of hole shapes according to theetch time. FIG. 1B illustrates the etch rate and the etch depth of thehole in relation to the etch time. FIG. 2A is a longitudinalcross-sectional view of a film arranged on a silicon wafer. FIG. 2Billustrates a relationship between the etch rate and the aspect ratio ofthe hole. FIG. 3 illustrates physical phenomena that occur upon etchingof the contact hole.

The required aspect ratio (AR) of a contact hole has typically beenaround 50. As illustrated in FIG. 2A, the aspect ratio (AR) isrepresented by the ratio of the depth h of the hole to the diameter φ ofthe hole opening (diameter at the top of the hole in the illustratedexample). Thus, when the hole depth h is 2 μm and the hole diameter φ is40 nm, the aspect ratio is 50.

In recent years, with the development of microfabrication technology,there has been a growing demand for holes having a diameter φ of about20 nm. In terms of the aspect ratio (AR), for example, holes having anaspect ratio of about 100 are now in demand.

Also, in etching a deep hole with a small diameter, the etch rate tendsto decrease as the depth of the hole to be etched increases. The etchrate corresponds to an amount that can be etched per unit time. Forexample, the etch rate may be represented by the etch amount per minute(nm/min).

For example, FIG. 1A illustrates a cross-sectional view of hole shapesaccording to the etch time upon etching holes in a silicon oxide (SiO₂)film. In this experiment, a lower side dual frequency type parallelplate etching apparatus that performs parallel plate plasma etching(e.g. FIG. 8) is used as an example. The etching process conditions areas follows: pressure 2.66 Pa, frequency 40 MHz and power density 1.18(W/cm²) for plasma generation high frequency power HF (High Frequency),frequency 3.2 MHz and power density 4.42 (W/cm²) for ion attraction highfrequency power LF (Low Frequency), and gas mixture of C₄F₆/C₄F₈/Ar/O₂as the processing gas. The silicon oxide film is etched under the aboveprocess conditions using a polysilicon mask. FIG. 1A illustrates thecross-sectional shapes of the holes, the hole diameters p, and theaspect ratios AR at etch times of 60 seconds, 180 seconds, 360 seconds,480 seconds, and 600 seconds. According to the experimental results, asthe etch depth h increases, the hole diameter φ increases and the aspectratio AR increases.

FIG. 1B [a] is a graph having a horizontal axis representing the etchtime ET and a vertical axis representing the etch depth. FIG. 1B [a]illustrates the relationship between the etch time ET and the depth h ofthe holes etched in the silicon oxide film and the remaining amount ofthe polysilicon mask, which has a predetermined etch pattern formedthereon and is used for etching the silicon oxide film. FIG. 1B [b] is agraph having a horizontal axis representing the etch time ET and avertical axis representing the etch rate E/R. FIG. 1B [b] illustratesthe etch rate of the silicon oxide film and the etch rate of the maskrelative to the etch time ET.

It can be appreciated from the graphs [a] and [b] of FIG. 1B that as theetch time ET becomes longer and the etched hole becomes deeper, the etchrate of the silicon oxide film decreases and it becomes increasinglydifficult to etch the hole bottom. Also, it can be appreciated that theetch rate of the mask is more or less constant irrespective of the etchtime ET, and the decrease rate of the polysilicon mask is more or lessconstant.

Based on the above, when an etching process is performed on a siliconoxide film having the same film properties throughout the depthdirection, an etch stop ultimately occurs. Also, when process conditionssuch as the pressure is changed in order to increase the etch rate, thedevice shape may be degraded.

FIG. 2B is a graph illustrating the etch rate of a silicon oxide filmand the etch rate of a mask in relation to the aspect ratio (horizontalaxis) in an exemplary case of etching a silicon oxide (SiO₂) film 92that is arranged on a silicon wafer 94 using a polysilicon mask 90 (seeFIG. 3). As illustrated in FIG. 2B, the etch rate of the silicon oxide(SiO₂) film 92 that is arranged on the silicon wafer 94 is proportionalto the aspect ratio (the etch rate of the silicon oxide film 92 dependson the aspect ratio). It can be appreciated from this result that as theetch time of FIG. 1A progresses and the aspect ratio increases, the etchrate of the silicon oxide film 92 decreases.

Also, it can be predicted based on the graph of FIG. 2B that an etchstop will occur at the hole bottom when the aspect ratio of the etchedhole increases to approximately 55. Note that because the polysiliconmask 90 is formed above the hole formed in the silicon oxide film 92,the etch rate of the polysilicon mask 90 does not depend on the aspectratio.

In the following, a reason why the etch rate decreases as the holebecomes deeper is explained with reference to FIG. 3. FIG. 3schematically illustrates physical phenomena of CF* radicals and ions ina case where deep holes are formed using a fluorocarbon (CF) based gasas an etching gas.

Physical phenomena of radicals include radical shadowing and Knudsentransport of radicals, which are each described below. In a vacuum, onlyCF* radicals (fluorocarbon radicals) in plasma that are at apredetermined angle of incidence enter an etched hole (see left sidedrawing of FIG. 3). CF* radicals at angles greater than thepredetermined angle cannot enter the hole. The predetermined angle isdetermined by the depth of the hole. As the depth of the hole increases,the angle of the CF* radicals that can reach the bottom of the holebecomes more limited, and the number of CF* radicals that reach the holebottom decreases. Such a phenomenon is referred to as radical shadowing.

Also, a large portion of the CF* radicals that have entered the holecollide with the wall surface of the hole. Such CF* radicals eitherstick to the wall surface or bounce back to proceed deeper into thehole. The Knudsen transport determines the probability of the collidinggas particles remaining stuck to the wall surface of the hole and theprobability of the gas particles bouncing back from the wall surface andproceeding toward the hole bottom to reach the hole bottom based on anattraction coefficient.

Similarly, only ions in plasma that are at a predetermined angle ofincidence enter the hole (see right side drawing of FIG. 3). Ions atangles greater than the predetermined angle cannot enter the hole. Thepredetermined angle is determined by the depth of the hole. As the holebecomes deeper, the predetermined angle of the ions that can enter thehole becomes more limited, and the number of ions that can reach thehole bottom decreases. Such a phenomenon is referred to as ionshadowing.

Ions have positive charges. Thus, when the positive charges areaccumulated at the bottom of the hole, they react against ions thatenter the hole. As a result, the entering ions are rebound from thebottom of the hole. Such a phenomenon is referred to as an ion chargeeffect. Owing to the motion of the radicals and ions of plasma in thevacuum, the number of radicals and ions of the plasma that reach thebottom of the hole decreases as the depth of the hole increases and theetch rate decreases as a result. Note that the etch rate may also beaffected by the type of radicals, the material of the etched film inwhich the hole is formed, compatibility between the radicals and thefilm material, and the temperature, for example.

The etch rate is expressed by the following equation (1). In thefollowing, factors affecting the etch rate are described in greaterdetail based on equation (1).

E/R=A×[E _(i)·Γ_(ion)]×[Γ_(radical)]/([E_(i)·Γ_(ion)]+[Γ_(radical)])  (1)

In the above equation (1), A represents a value determined by theproperty and type of material, and E_(i) represents the ion energy.Γ_(ion) represents the number of ions per unit area, and Γ_(radical)represents the number of radicals per unit area.

The etch rate is affected by the sputter yield, which represents thenumber of molecules that leave a substance when an ion collides with thesubstance. The sputter yield has a correlation with the ion energy.Thus, in equation (1), the ion energy E_(i) is used instead of thesputter yield to represent the etch rate of ions. On the other hand, theenergy of radicals is lower than that of ions, and as such, the etchrate of radicals is not taken into account in equation (1).

Thus, based on the above equation (1), the etch rate may be determinedprimarily based on the number of ions and the number of radicals perunit area and the ion energy. Accordingly, in the embodiments describedbelow, the material of the etched film is altered in the depth directionin which the etch rate decreases. In this way, a decrease in the numberof radicals and ions that reach the bottom of the hole as the holebecomes deeper may be prevented, and as a result, a decrease in the etchrate may be suppressed.

[Film Structure]

In the following, an exemplary layer structure of a film that may beused in an embodiment of the present invention is described withreference to FIG. 4. In the present example, a first silicon oxide film12 and a second silicon oxide film 14 are stacked in this order on asilicon wafer 10 made of silicon (Si), and a mask 16 is formed on thesecond silicon oxide film 14. The mask 16 is a polysilicon mask having apredetermined pattern formed thereon according to a desired etch shape.In the present embodiment, the silicon oxide film is arranged into atwo-layer structure including the first silicon oxide film 12 and thesecond silicon oxide film 14. However, the present invention is notlimited to such an embodiment, and the silicon oxide film mayalternatively have three or more layers, for example. Also, the siliconoxide film may be made of one single layer having a silicon content perunit volume and/or a volume density that varies in the depth direction,for example.

In the present embodiment, the second silicon oxide film 14 is made of adense SiO₂ film (hard SiO₂), and the first silicon oxide film 12 is madeof a relatively coarser SiO₂ film (soft SiO₂) compared to the secondsilicon oxide film 14. In this way, the silicon content per unit volumeand the volume density of the second silicon oxide film 14 correspondingto the upper layer is arranged to be relatively higher than the siliconcontent per unit volume and the volume density of the first siliconoxide film 12 corresponding to the lower layer.

The second silicon oxide film 14 may be made of a TEOS (tetraethylorthosilicate) film or a USG (undoped silicate glass) film, for example.As is described in detail below, the second silicon oxide film 14 may beformed using various film forming methods. For example, the TEOS filmmay be formed by plasma CVD (chemical vapor deposition) using a TEOSgas, and the USG film may be formed by plasma CVD using a TEOS gas andan oxygen gas. The first silicon oxide film 12 may be a BPSG (boronphosphate silicate glass) film, which is a silicon oxide film doped withB (boron) and P (phosphorus), for example. In this case, because thesecond silicon oxide film 14 corresponding to the upper layer is notdoped with B (boron) and P (phosphorus), the second silicon oxide film14 may be arranged to have a relatively higher silicon content per unitvolume compared to the first silicon oxide film 12.

Also, the first silicon oxide film 12 may be a fluorine-doped siliconoxide (SiOF) film, for example. In this case, because the second siliconoxide film 14 corresponding to the upper layer does not containfluorine, the second silicon oxide film 14 may be arranged to have arelatively higher silicon content per unit volume compared to the firstsilicon oxide film 12 corresponding to the lower layer. Also, in thiscase, the first silicon oxide film 12 corresponding to the lower layermay be arranged to have a relatively higher fluorine content per unitvolume compared to the second silicon oxide film 14 corresponding to theupper layer.

Also, the first silicon oxide film 12 may be formed using the SOG(spin-on-glass) coating technique, for example. Also, the first siliconoxide film 12 may be a FSG (fluorosilicate glass) film as described indetail below.

As illustrated in FIG. 4, in the present embodiment, a plasma isgenerated from an etching gas containing a fluorocarbon (CF) based gas,and a predetermined pattern is etched in the first silicon oxide film 12and the second silicon oxide film 14 by the generated plasma usingpolysilicon that is arranged on the second silicon oxide film 14 as themask 16.

As can be appreciated from the following chemical formula (2), duringetching, the silicon oxide film and fluorocarbons react with each otherto generate silicon fluoride and carbon dioxide. Carbon dioxide andsilicon tetrafluoride are gases that are discharged to the exterior. Inthis way, the silicon oxide film is etched.

SiO₂+CF₄→SiF₄↑(gas)+CO₂↑(gas)  (2)

In the following, the relationship between the ratio of carbon (C) tofluorine (F) contained in plasma and the etch rate E/R of the siliconoxide film are described with reference to FIG. 5. At the center portionof the graph of FIG. 5 where the ratio of carbon (C) to fluorine (F)within the plasma has a good balance, silicon (Si) and oxygen (O) withinthe silicon oxide film are encouraged to react with the plasma to promptthe above reaction SiF₄↑+CO₂↑, and because the above reaction isencouraged, the etch rate E/R of the silicon oxide film increases. Onthe other hand, at the left side portion of the graph of FIG. 5 wherethe content of carbon (C) is lower than the content of fluorine (F)within the plasma, the generation of CO₂↑ (see FIG. 6) of the abovereaction SiF₄↑+CO₂↑ is not adequately encouraged due to the shortage ofcarbon (C). As a result, etching the silicon oxide film becomes moredifficult and the etch rate E/R of the silicon oxide film decreases.Similarly, at the right side portion of the graph of FIG. 5 where thecontent of fluorine (F) within the plasma is lower than the content ofcarbon (C), the generation of SiF₄↑ (see FIG. 6) of the above reactionSiF₄↑+CO₂↑ is not adequately encouraged. As a result, etching of thesilicon oxide film becomes more difficult and the etch rate E/R of thesilicon oxide film decreases. As can be appreciated, the etch rate E/Rof the silicon oxide film is affected by the composition elements of theplasma, and for example, the etch rate E/R decreases when the amount offluorine radicals within the plasma decreases.

Referring to FIG. 6, it can be appreciated that the silicon oxide filmcan be etched with a smaller amount of fluorine radicals as the contentof silicon within the silicon oxide film becomes lower. As describedabove, the amount of fluorine radicals that reach the bottom of a holedecreases as the hole being etched in the silicon oxide film becomesdeeper. Thus, as the amount of fluorine radicals that reach the bottomof the hole decreases, the etch rate decreases. In the presentembodiment, the composition of the silicon oxide film is varied in thedepth direction. That is, in the present embodiment, as a hole is etcheddeeper, the silicon content and/or volume density of the silicon oxidefilm at the bottom of the hole is arranged to decrease. Accordingly, byimplementing the silicon oxide film structure of the present embodiment,even when the amount of fluorine radicals that reach the bottom of thehole decreases as the hole etched in the silicon oxide film becomesdeeper, a decrease in the etch rate may be suppressed, and an etch stopmay be avoided.

In the silicon oxide film used in the present embodiment, the siliconcontent and/or volume density of the second silicon oxide film 14corresponding to the upper layer is arranged to be relatively higherthan the silicon content and/or volume density of the first siliconoxide film 12 corresponding to the lower layer. In this way, even when adeep hole is formed in the first silicon oxide film 12 corresponding tothe lower layer, because the first silicon oxide film 12 is a coarsersilicon oxide film compared to the second silicon oxide film 14corresponding to the upper layer, a decrease in the etch rate may besuppressed.

In the following, an exemplary film forming method is described that maybe used in the present embodiment for forming the silicon oxide filmhaving the upper layer second silicon oxide film 14 made of a dense filmand the lower layer first silicon oxide film 12 made of a relativelycoarser film. In this case, the silicon content and volume density ofthe dense film is higher than the silicon content and volume density ofthe relatively coarser film. To form such a silicon oxide film, a SOG(spin-on-glass) film may be formed on the silicon wafer 10 as the firstsilicon oxide film 12 using the SOG (spin-on-glass) coating technique.Then, a USG film may be formed as the second silicon oxide film 14 usinga plasma CVD method. Then, a polysilicon layer may be deposited in asimilar manner using a CVD method to form a patterned photoresist, andthe polysilicon mask 16 may be formed by dry etching. Then, thephotoresist may be removed by dry etching using an O₂ plasma, forexample.

[Modification 1]

As another exemplary method for forming the silicon oxide film with theupper layer second silicon oxide film 14 having a lower silicon contentthan the silicon content of the lower layer first silicon oxide film 12,the first silicon oxide film 12 may be doped with impurities such asboron (B) and/or phosphorus (P) while being formed. In this way, thefirst silicon oxide film 12 having the impurities doped therein may havea lower silicon content compared to that of the second silicon oxidefilm 14 that does not have any impurities doped therein. Accordingly,the silicon content of the lower layer first silicon oxide film 12 maybe arranged to be lower than the upper layer second silicon oxide film14, and as a result, even when a fewer number of fluorine radicals reachthe bottom of a deep hole formed in the lower layer first silicon oxidefilm 12, a decrease in the etch rate may be suppressed.

An exemplary method of forming the lower layer silicon oxide film havingimpurities doped therein may involve forming a silicade glass film suchas a BPSG film as the first silicon oxide film 12 on the silicon wafer10 using a high density plasma CVD method. Then, a TEOS film may beformed as the second silicon oxide film 14 under a low pressure statethat is slightly lower than ordinary pressure (a pressure higher than alow vacuum state) using a CVD method. In this case, because impuritiesare doped in the lower layer first silicon oxide film 12, the siliconcontent of the lower layer first silicon oxide film 12 may be lower thanthat of the upper layer second silicon oxide film 14. Then, apolysilicon layer may be deposited using a CVD method to form apatterned photoresist, and the polysilicon mask 16 may be formed by dryetching. Then, the photoresist may be removed by dry etching using an O₂plasma, for example.

[Modification 2]

As another method of varying the silicon content per unit volume of thesilicon oxide film in the depth direction, the fluorine content per unitvolume of the silicon oxide film may be varied in the depth direction.That is, in the case where the silicon oxide film has a two-layerstructure, the fluorine content of the lower layer first silicon oxidefilm 12 may be arranged to be higher than the fluorine content of theupper layer second silicon oxide film 14. In this way, by having thelower layer first silicon oxide film 12 have a higher content offluorine (F) than the upper layer second silicon oxide film 14, evenwhen a fewer number of fluorine F radicals reach the bottom of a deephole formed in the lower layer first silicon oxide film 12, fluorine (F)may be supplied from the first silicon oxide film 12 such that adecrease in the etch rate may be suppressed. Note that in someembodiments, the silicon oxide film may be formed using a single filmforming method and arranged into a multilayer structure having afluorine content that gradually increases in the depth direction. Inthis way, the lower layer silicon oxide films may be arranged to have arelatively higher fluorine content than the upper layer silicon oxidefilms, and as such, the lower layer silicon oxide films may be arrangedto have a relatively lower silicon content than the upper layer siliconoxide films.

Further, a silicon oxide film combining the aspects of the aboveembodiment and Modification 1, a silicon oxide film combining theaspects of the above embodiment and Modification 2, or a silicon oxidefilm combining the aspects of Modification 1 and Modification 2 may beused, for example. Note that a silicon oxide film having otherconfigurations may also be used.

Note that in the two-layer silicon oxide film described above, thesilicon/fluorine content within each layer may be uniform, or thesilicon/fluorine content within each layer may be arranged to vary inthe depth direction. For example, the upper layer second silicon oxidefilm 14 may be arranged to have a higher silicon content than the lowerlayer first silicon oxide film 12, and the lower layer first siliconoxide film 12 may be arranged to have a higher fluorine content than theupper layer second silicon oxide film 14.

Also, the silicon oxide film may have an impurity content that varies inthe depth direction. For example, the lower layer first silicon oxidefilm 12 may be arranged to have a lower impurity content than the upperlayer second silicon oxide film 14. Note that the impurity contentwithin each layer may be uniform, or the impurity content within eachlayer may also be arranged to vary in the depth direction.

[Switching Section of Silicon Oxide Film]

In the following, a switching section where the layers of the siliconoxide film are switched between the first silicon oxide film 12 and thesecond silicon oxide film 14 are described with reference to FIG. 7.FIG. 7 illustrates an example in which an etching gas containing afluorocarbon (CF) based gas is supplied upon etching the upper layersecond silicon oxide film 14, and a fluorine (F) gas is supplied inaddition to the etching gas containing a fluorocarbon (CF) based gasupon etching the lower layer first silicon oxide film 12.

In the graph of FIG. 7, the horizontal axis represents the aspect ratioand the vertical axis represents the etch rate of the silicon oxidefilm. Note that in the graph of FIG. 7, “first etch rate” represents theetch rate upon etching the first silicon oxide film 12 (hard SiO₂), and“second etch rate” represents the etch rate upon etching the secondsilicon oxide film 14 (coarse silicon oxide film such as afluorine-containing film or a SOG film).

In order to solve the etch stop problem as described above, measuresneed to be implemented to prevent the first etch rate from becomingzero. Thus, the silicon oxide film layers are switched from the upperlayer second silicon oxide film 14 to the lower layer first siliconoxide film 12 such that when forming a hole using an etching methodaccording to the present embodiment, the etch rate may be switched fromthe first etch rate to the second etch rate at a point where the aspectratio of the hole being etched reaches 50 or a value less than 55, whichcorresponds to the value at which the first etch rate becomes zero. Thatis, the upper layer second silicon oxide film 14 is arranged at an upperportion of the silicon oxide film extending down to where the aspectratio is 50, and the lower layer first silicon oxide film 12 is arrangedat a lower portion of the silicon oxide film where the aspect ratio isgreater than 50.

Note, however, that in consideration of productivity issues, in apreferred embodiment, the switching section for switching from the upperlayer second silicon oxide film 14 to the lower layer first siliconoxide film 12 may be arranged to be where the aspect ratio reaches 40and beyond such that the etch rate may be prevented from falling below100 nm/min, for example.

[Etching Apparatus]

In the following, an exemplary etching apparatus 130 according to anembodiment of the present invention that may be used to form a hole inthe layered film as illustrated in FIG. 4 is described with reference toFIG. 8.

The etching apparatus 130 includes a chamber C (processing chamber) thatis maintained airtight and is electrically grounded. The etchingapparatus 130 is connected to a gas supply source 120. The gas supplysource 120 supplies an etching gas containing a fluorocarbon (CF) basedgas as an etching gas. The fluorocarbon gas may containhexafluoro-1,3-butadiene (C₄F₆) gas, for example.

The chamber C is cylindrical and may be made of aluminum having analumite-treated (anodized) surface, for example. A mounting table 102for supporting a silicon wafer W is arranged inside the chamber C. Themounting table 102 also functions as a lower electrode. The mountingtable 102 is supported by a conductive support 104 and is arranged to bemovable up and down by an elevating mechanism 107 via an insulatingplate 103. The elevating mechanism 107 is arranged in the chamber C andis covered by a bellows 108 made of stainless steel. A bellows cover 109is arranged around the outer side of the bellows 108. A focus ring 105,which may be made of monocrystalline silicon, for example, is arrangedat an upper side outer periphery portion of the mounting table 102.Further, a cylindrical inner wall member 103 a, which may be made ofquartz, for example, is arranged around the mounting table 102 and thesupport 104.

A first high frequency power supply 110 a is connected to the mountingtable 102 via a first matching unit 111 a, and the first high frequencypower supply 110 a is configured to supply a high frequency power of apredetermined frequency (e.g. 40 MHz) for plasma generation. Also, asecond high frequency power supply 110 b is connected to the mountingtable 102 via a matching unit 111 b, and the second high frequency powersupply 110 b is configured to supply a high frequency power of apredetermined frequency (e.g. 3.2 MHz) for biasing. Also, a shower head116 is arranged above the mounting table 102 facing opposite andparallel to the mounting table 102 such that the shower head 116 mayfunction as an upper electrode. In this way, the shower head 116 and themounting table 102 may function as a pair of electrodes.

An electrostatic chuck 106 for electrostatically attracting a siliconwafer W is arranged on an upper surface of the mounting table 102. Theelectrostatic chuck 106 includes an electrode 106 a arranged within aninsulator 106 b. A DC voltage source 112 is connected to the electrode106 a, and when a DC voltage from the DC voltage source 112 is appliedto the electrode 106 a, the silicon wafer W is attracted to theelectrostatic chuck 106 by a Coulomb force.

A coolant path 104 a is formed within the support 104. A coolant inletpipe 104 b and a coolant outlet pipe 104 c are connected to the coolantpath 104 a. By circulating a suitable coolant such as cooling waterwithin the coolant path 104 a, the silicon wafer W may be controlled toa predetermined temperature. Also, a pipe 130 for supplying heattransfer gas (backside gas) such as helium (He) gas is arranged at theback side of the silicon wafer W.

The shower head 116 is arranged at a ceiling portion of the chamber C.The shower head 116 includes a main body 116 a and an upper plate 116 bforming an electrode plate. The shower head 116 is supported at a topportion of the chamber C via an insulating member 145. The main body 116a is made of a conductive material such as aluminum having analumite-treated (anodized) surface, for example, and is configured todetachably support the upper plate 116 b thereunder.

A diffusion chamber 126 a for diffusing gas is arranged within the mainbody 116 a, and a plurality of gas passage holes 116 d are formed at thebottom of the main body 116 a at positions below the diffusion chamber126 a. A plurality of gas introducing holes 116 e penetrating throughthe upper plate 116 b in its thickness direction and communicating withthe gas passage holes 116 d are arranged at the upper plate 116 b. Inthis way, gas that is supplied to the diffusion chamber 126 a may beshowered into a plasma processing space within the chamber C via the gaspassage holes 116 d and the gas introducing holes 116 e. Note that apipe (not shown) for circulating a coolant may be arranged within themain body 116 a, for example, in order to cool and adjust the showerhead 116 to a desired temperature.

A gas inlet 116 g for introducing gas into the diffusion chamber 126 ais formed at the main body 116 a. The gas supply source 120 is connectedto the gas inlet 116 g.

A variable DC voltage source 152 is electrically connected to the showerhead 116 via a low-pass filter (LPF) 151. Power supply operations of thevariable DC voltage source 152 may be turned on/off by an on/off switch153. The on/off switch 153 may be turned on as is necessary when highfrequencies from the first high frequency power supply 110 a and thesecond high frequency power supply 110 b are to be applied to themounting table 102 and plasma is to be generated within the plasmaprocessing space of the chamber C, for example. In this way, apredetermined DC voltage may be applied to the shower head 116.

A cylindrical grounding conductor 101 a is arranged to extend above theheight of the shower head 116 from a side wall of the chamber C. Thecylindrical grounding conductor 101 a has a top plate arranged thereon.An exhaust port 171 is arranged at the bottom of the chamber C. Anexhaust device 173 is connected to the exhaust port 171. The exhaustdevice 173 includes a vacuum pump, and the pressure within the chamber Cmay be reduced to a predetermined vacuum level by operating the vacuumpump. Also, a gate valve 175 is arranged at the side wall of the chamberC for opening/closing a transfer port 174 to enable loading/unloading ofthe silicon wafer W into/out of the chamber C via the transfer port 174.

A dipole ring magnet 124 is arranged to extend annularly orconcentrically around the chamber C and vertically across the positionof the mounting table 102 during processing.

With such a configuration, a RF electric field in the vertical directionmay be formed by the first high frequency power supply 110 a and ahorizontal magnetic field may be formed by the dipole ring magnet 124within a space between the mounting table 102 and the shower head 116.By prompting magnetron discharge using the above orthogonalelectromagnetic fields, a plasma may be generated at a high densityaround the surface of the mounting table 102.

For example, the two-layer silicon oxide film formed in theabove-described manner may be subject to parallel plate plasma etchingin a lower side dual frequency parallel plate etching apparatus (e.g.FIG. 8) under the following process conditions. For example, thepressure within the processing chamber (chamber C) may be set to 2.66Pa, high frequency powers with frequencies of 40 MHz/3.2 MHz and powerdensities of 1.18/4.42 (W/cm²) may be applied to the lower electrode,plasma of a processing gas of C₄F₆/C₄F₈/Ar/O₂ may be generated, and thesilicon oxide film may be etched using polysilicon as a mask. In thiscase, the etch depth may be relatively deep, but as described inconnection with FIG. 7, the etch depth may reach the first silicon oxidefilm 12 (e.g. SOG film) having a relatively lower silicon contentcompared to the second silicon oxide film 14 (e.g. USG film) at a pointbefore the aspect ratio reaches a value at which the etch rate decreasesto zero to cause an etch stop. In this way, a decrease in the etch ratemay be suppressed, and a contact hole with a relatively high aspectratio of 60 or greater may be formed, for example.

[Film Forming Apparatus]

In the following, exemplary film forming apparatuses are described thatmay be used to form the layered silicon oxide film including the firstsilicon oxide film 12 and the second silicon oxide film 14 asillustrated in FIG. 4.

Example 1

In Example 1, a SOG film is formed as the first silicon oxide film 12,and a USG film is formed as the second silicon oxide film 14 by plasmaCVD. FIG. 9 is a longitudinal cross-sectional view of a film formingapparatus 140 used to form the SOG film of Example 1. FIG. 10 is a planview of the film forming apparatus of FIG. 9.

The film forming apparatus 140 that is used to form the SOG film as thefirst silicon oxide film 12 is configured to apply a SOG coatingsolution on the surface of the wafer W using a spin coating technique.As illustrated in FIGS. 9 and 10, the film forming apparatus 140includes a spin chuck 48 that is rotated by a motor M within a cup 46, aprocess liquid supply nozzle 50 arranged at a tip portion of a processliquid supply pipe 50A, a rinsing liquid supply nozzle 52 arranged at atip portion of a rinsing liquid supply pipe 52A, a movable arm 56 thatis configured to hold the nozzles 50 and 52 and scan a wafer W acrossits radial directions while moving along a guide rod 54, a processliquid nozzle waiting unit 56A and a dummy dispense unit 56B at whichthe process liquid supply nozzle 50 is placed while waiting, a rinsingliquid nozzle waiting unit 60 at which the rinsing liquid supply nozzle52 is placed while waiting, and an exhaust pipe 62.

In the following, a coating process that is performed at the filmforming apparatus 140 is described. When the wafer W that is mounted onthe spin chuck 48 is rotated along with the spin chuck 48, the processliquid supply nozzle 50 is held by the movable arm 56 and is moved abovethe wafer W such that droplets of SOG solution corresponding to aprocessing liquid may be dropped on the wafer W. The SOG solution may bea mixture of the film material such as a silanol compound and a solventsuch as ethyl alcohol, for example. At this point, the wafer W is beingrotated at a relatively high speed (2000-6000 rpm), and as a result, theSOG solution is diffused from a center portion to a periphery portion ofthe wafer W by a centrifugal force to form the SOG film on the wafer W.After the SOG film is formed, the rinsing liquid supply nozzle 52 ismoved above the wafer W, and the SOG film formed on the peripheralportion of the wafer W is dissolved and removed by the rinsing liquid,which may be an ethyl alcohol solution, for example. Then, after thesolvent is evaporated by a heating process at a temperature of 100-140°C. in a pre-heating step, the wafer W is loaded into a heating apparatus(not shown), and a heating process is performed on the wafer W at atemperature of about 400-450° C. such that siloxane bonding occurswithin the SOG film. In this way, the SOG film corresponding to asilicon oxide film is formed. Note that in the case of forming the SOGfilm to have a predetermined film thickness, the process steps ofapplying the SOG solution on the wafer W and evaporating the solvent maybe repeated multiple times after which the wafer W may be loaded intothe heating apparatus to perform the heating process, for example.

The two-layer silicon oxide film formed in the above-described mannermay be subject to parallel plate plasma etching in a lower side dualfrequency parallel plate etching apparatus (e.g. FIG. 8) under thefollowing process conditions. For example, the pressure may be set to2.66 Pa, high frequency powers with frequencies of 40 MHz and 3.2 MHzmay be applied at power densities of 1.18 (W/cm²) and 4.42 (W/cm²),respectively, and the silicon oxide film may be etched by a processinggas of C₄F₆/C₄F₈/Ar/O₂ using polysilicon as a mask. In this case, whenthe etch depth reaches a certain depth, the silicon oxide film layer isswitched to the first silicon oxide film 12 (e.g. SOG film) having arelatively lower silicon content compared to the second silicon oxidefilm 14 (e.g. USG film). In this way, a decrease in the etch rate may besuppressed, and a contact hole with a relatively high aspect ratio of 60or greater may be formed, for example.

Example 2

In Example 2, a silicade glass film such as a BPSG film is formed as thefirst silicon oxide film 12 using a high density plasma CVD method, anda TEOS film is formed as the second silicon oxide film 14 under a lowpressure state that is slightly lower than ordinary pressure (a pressurehigher than a low vacuum state) using a CVD method. FIG. 11 is alongitudinal cross-sectional view of a film deposition apparatus 301that may be used to form the BPSG film of Example 2.

As illustrated in FIG. 11, the film deposition apparatus 301 that isused to form the BPSG film as the first silicon oxide film 12 includesan upper electrode 203 and a lower electrode 202 of a parallel platesystem arranged within a chamber 201. The film deposition apparatus 301also includes an RF power supply 207 for supplying an RF power at afrequency of 13.56 MHz between the upper electrode 203 and the lowerelectrode 202 to convert a film forming gas into plasma. The pressurewithin the chamber 201 is reduced to a suitable pressure by an exhaustdevice that is connected to an exhaust port 205. The lower electrode 202also functions as a mounting table on which a silicon wafer 310 may bemounted, and a heating unit 206 for heating the silicon wafer 310 isarranged at the lower electrode 202.

Further, the film deposition apparatus 301 includes a supply source 21for supplying phosphorus acid dimethoxy trimethylsilylester (hereinafterreferred to as SOP-11(b)) as a phosphorus-containing compound source 33,a supply source 22 for supplying SiH₄ as a silicon-containing compoundsource 36, a supply source for supplying oxygen (O₂) gas as an oxidativegas source, and a supply source for supplying Ar or N₂ as a rare gassource. These supply sources are connected to a gas introducing port 204of the chamber 201 via a pipe 24 e.

The RF power supply 207 is connected to the upper electrode 203 via amatching circuit 208. Also, in FIG. 11, pipes 24 a-24 d for guidinggases are respectively connected to flow meters 25 a-25 d. Also, valves26 a-26 d are arranged at the pipes 24 a-24 d for opening/closing theflow paths for the source gases formed in the pipes 24 a-24 d. Further,adjusting units 32 and 35, which may be a heater or a cooler, forexample, may be provided for controlling the temperature of the sources33 and 36.

In the present example, first, the silicon wafer 310 is introduced intothe chamber 201 of the film deposition apparatus 301. Then, the siliconwafer 310 is heated and the temperature of the silicon wafer 310 ismaintained within a predetermined temperature range according to filmforming conditions. Note that a base insulating film made of a siliconoxide film, for example, may be formed on the silicon wafer 310.

Then, a film forming gas that is adjusted to be within a rangeprescribed by the film forming conditions is introduced into the chamber201, and the film forming gas is converted into plasma by an RF powerprescribed by the film forming conditions.

By maintaining the above state for a predetermined time period, a PSGfilm having a high concentration of phosphorus (phosphorus-containinginsulating film) may be formed at a predetermined film thickness. Notethat the PSG film may be fluidized at around the temperature prescribedby the film forming conditions, and in this case, planarization may beachieved along with the film formation.

Otherwise, a heating process may be separately performed forplanarization after forming the PSG film (phosphorus-containinginsulating film) on the silicon wafer 310 so that the PSG film may befluidized and flattened. In this way, a PSG film having a flattenedsurface may be formed.

According to the present example, SiH₄ is used as a silicon-containingcompound, SOP-11(b) is used as a phosphorus-containing compound, and asuitable amount of oxidative gas such as oxygen (O₂) gas is added. Inthis way, the fluidization temperature for enabling planarization may besubstantially reduced.

In the following, a method of forming a BPSG film containing phosphorususing a plasma excitation method is described as an exemplary devicefabrication method implemented by the film deposition apparatus 301.

In the present example, a gas mixture containing SiH₄, SOP-11(b), TMB orTEM, and N₂O is used as the film forming gas, and the BPSG film isformed under the following film forming conditions.

Temperature 150-400° C. Gas Pressure 0.5-3.0 Torr (66.66-399.9 Pa) FlowRate of SOP-11(b) Bubbling Gas 300-1500 sccm (N₂ or Ar) SOP-11(b) SourceTemperature 45° C. Flow Rate of SiH₄ 100-2000 sccm Flow Rate of TMP15-600 sccm Flow Rate of TMB or TEB 10-300 sccm Flow Rate of OxidativeGas (O₂) 300 sccm or less RF Power 0.147-1.18 W/cm² Frequency 100kHz-2.5 GHz

By implementing the above film forming conditions, a BPSG film(phosphorus-containing insulating film) made of a mixture of SiO₂,phosphorus (P), and boron (B) may be formed on the silicon wafer 310.

The two-layer silicon oxide film formed in the above-described mannermay be subject to parallel plate plasma etching in a lower side dualfrequency parallel plate etching apparatus (e.g. FIG. 8) under thefollowing process conditions. For example, the pressure may be set to2.66 Pa, high frequency powers with frequencies of 40 MHz and 3.2 MHzmay be applied at power densities of 1.18 (W/cm²) and 4.42 (W/cm²),respectively, and the silicon oxide film may be etched by a processinggas of C₄F₆/C₄F₆/Ar/O₂ using polysilicon as a mask. In this case, whenthe etch depth reaches a certain depth, the silicon oxide film layer isswitched to the first silicon oxide film 12 (e.g. BPSG film) having arelatively lower silicon content compared to the second silicon oxidefilm 14 (e.g. TEOS film). In this way, a decrease in the etch rate maybe suppressed, and a contact hole with a relatively high aspect ratio of60 or greater may be formed, for example.

Note that in the above Example 1, the first silicon oxide film 12 ismade of a SOG film, and the second silicon oxide film 14 is made of aUSG film. In the above Example 2, the first silicon oxide film 12 ismade of a BPSG film, and the second silicon oxide film 14 is made of aTEOS film. However, the combination of the first silicon oxide film 12and the second silicon oxide film 14 may be altered in other examples.The TEOS film may be formed by plasma CVD using TEOS gas, and the USGfilm may be formed by plasma CVD using TEOS gas and oxygen gas.

Example 3

In Example 3, the first silicon oxide film 12 varies from that ofExample 2. Specifically, in the present example, a FSG film is formed asthe first silicon oxide film 12 under the following process conditionsusing the above-described film deposition apparatus 301 (FIG. 11). TheFSG film forming conditions are as follows.

Temperature 150-400° C. Gas Pressure 533.288 × 10⁻³ Pa Flow Rate of SiF₄Gas 26 sccm Flow Rate of SiH₄ Gas 38 sccm Flow Rate of O₂ Gas 111 sccmFlow Rate of Ar Gas 60 sccm RF Power 2.16 W/cm²

By implementing the above film forming conditions, a FSG film may beformed on the silicon wafer 310. Note that in some embodiments, the gasratio of the SiF₄ gas to the SiH₄ gas may be arranged to vary in orderto vary the fluorine content of the FSG film. That is, the flow rate ofthe SiF₄ gas may be increased with respect to that of the SiH₄ gas suchthat the fluorine content of the FSG film may increase in the depthdirection.

In another example, a SiOF film may be formed as the first silicon oxidefilm 12 under the following process conditions using an inductivelycoupled plasma (ICP) apparatus.

Growth Furnace Pressure 533.288 × 10⁻³ Pa (4 mTorr) Flow Rate of SiliconFluoride (SiF₄) 20-26 sccm Gas to be Introduced Flow Rate of SiH₄ Gas tobe Introduced 38 sccm Flow Rate of O₂ Gas to be Introduced 111 sccm FlowRate of Ar Gas to be Introduced 60 sccm High Frequency (ICP) Power 3.93W/cm² Bias Power 2.16 W/cm²

By forming a fluorine-containing film as the first silicon oxide film12, when the etch depth reaches a certain depth in the two-layer siliconoxide film, for example, the silicon oxide film layer may be switched tothe first silicon oxide film 12 (FSG film) having a higher fluorinecontent than the second silicon oxide film 14 (e.g. USG film or TEOSfilm). In this way, when a deep hole is etched in the silicon oxidefilm, fluorine radicals may be more easily supplied to the bottom of thehole such that a decrease in the etch rate may be suppressed and acontact hole having a relatively high aspect ratio of 60 or greater maybe formed, for example.

Example 4

Instead of arranging the silicon oxide film into two layers includingthe first silicon oxide film 12 and the second silicon oxide film 14, amultilayer film may be formed by repetitively forming silicon oxidefilms using the film formation method of Example 3, for example. In thiscase, the flow rate of the SiF₄ gas may initially be set relatively highwith respect to the flow rate of the SiH₄ gas to form a silicon oxidefilm having a relatively high fluorine content at deeper portions of themultilayer film, and the flow rate of the SiF₄ gas may gradually bedecreased with respect to the flow rate of the SiH₄ gas.

Although certain illustrative embodiments of the present invention havebeen described above with reference to the accompanying drawings, thepresent invention is not limited to these embodiments. It would beapparent to those skilled in the art that numerous variations andmodifications may be made within the scope of the present invention inlight of the disclosures made herein. It is to be understood that all ofsuch variations and modifications are included within the scope of thepresent invention.

For example, although the FSG film and the BPSG film are formed by theplasma CVD film deposition apparatus of FIG. 11 in the above describedembodiments, these films may also be formed using the film formingapparatus of FIG. 9 that is used to form the SOG film. Also, the SiOFfilm may be formed using the above film forming apparatus used to formthe SOG film or the plasma CVD film deposition apparatus, for example.

Also, the workpiece subject to processing according to the presentinvention is not limited to the silicon wafer described above. Forexample, a large substrate for a flat panel display, or a substrate foran EL (electroluminescence) element or a solar cell may also be used asthe workpiece.

The present application is based on and claims the benefit of priorityof Japanese Patent Application No. 2012-141660 filed on Jun. 25, 2012and U.S. Provisional Application No. 61/667,527 filed on Jul. 3, 2012,the entire contents of which are herein incorporated by reference.

DESCRIPTION OF THE REFERENCE NUMERALS

-   10 silicon wafer-   12 first silicon oxide film-   14 second silicon oxide film-   16 mask-   130 etching apparatus-   140 film forming apparatus-   201 chamber-   301 film deposition apparatus

1. An etching method comprising the steps of: supplying an etching gascontaining a fluorocarbon (CF) based gas into a processing chamber andgenerating a plasma from the etching gas; and etching a silicon oxidefilm through a polysilicon mask using the plasma, the polysilicon filmhaving a predetermined pattern and being arranged on the silicon oxidefilm, and the silicon oxide film having at least one of a siliconcontent per unit volume, a fluorine content per unit volume, and avolume density that varies in a depth direction.
 2. The etching methodas claimed in claim 1, wherein at least one of the silicon content andthe volume density of the silicon oxide film decreases in the depthdirection.
 3. The etching method as claimed in claim 1, wherein thefluorine content of the silicon oxide film increases in the depthdirection.
 4. The etching method as claimed in claim 1, wherein thesilicon oxide film includes an upper layer silicon oxide film and alower layer silicon oxide film; and at least one of the silicon contentper unit volume and the volume density of the upper layer silicon oxidefilm is higher than at least one of the silicon content per unit volumeand the volume density of the lower layer silicon oxide film.
 5. Theetching method as claimed in claim 1, wherein the silicon oxide filmincludes an upper layer silicon oxide film and a lower layer siliconoxide film; and the fluorine content per unit volume of the lower layersilicon oxide film is higher than the fluorine content per unit volumeof the upper layer silicon oxide film.
 6. The etching method as claimedin claim 1, wherein the silicon oxide film has an impurity content thatvaries.
 7. The etching method as claimed in claim 1, wherein the siliconoxide film includes an upper layer silicon oxide film and a lower layersilicon oxide film; the upper layer silicon oxide film includes at leastone of a TEOS film and a USG film; and the lower layer silicon oxidefilm includes at least one of a BPSG film, a SiOF film, and a FSG film.8. The etching method as claimed in claim 4, wherein the silicon oxidefilm is configured to have the upper layer silicon oxide film switchedto the lower layer silicon oxide film at a point where an aspect ratioof a hole etched by the etching method is less than or equal to 50, theaspect ratio representing a ratio of a depth of the hole with respect toa diameter of an opening of the hole.
 9. The etching method as claimedin claim 8, wherein the silicon oxide film is configured to have theupper layer silicon oxide film switched to the lower layer silicon oxidefilm at a point where an aspect ratio of a hole etched by the etchingmethod is less than or equal to 40, the aspect ratio representing aratio of a depth of the hole with respect to a diameter of an opening ofthe hole.
 10. The etching method as claimed in claim 1, wherein thefluorocarbon based gas contains hexafluoro-1,3-butadiene (C₄F₆) gas. 11.The etching method as claimed in claim 1, wherein the silicon oxide filmis formed using at least one of a CVD (chemical vapor deposition)technique and a SOG (spin-on glass) technique.
 12. An etching apparatuscomprising: a processing chamber; and a gas supply source; wherein theprocessing chamber accommodates a workpiece including a silicon oxidefilm and a polysilicon mask, which has a predetermined pattern and isarranged on the silicon oxide film, the silicon oxide film having atleast one of a silicon content per unit volume, a fluorine content perunit volume, and a volume density that varies in a depth direction; thegas supply source supplies an etching gas containing a fluorocarbon (CF)based gas into the processing chamber; and the silicon oxide film isetched by a plasma generated from the etching gas through thepolysilicon mask having the predetermined pattern.